The friction materials of the above type include five classes of components: a fibrous material base, a binder and a filler, one or more lubricants or friction modifiers, one or more abrasives. To a great extent asbestos was used in the past as the fibrous material, which material however presents considerable environmental problems and has well known toxic effects on human health and for this reason has been banned by legislation for a long time. This material has therefore been replaced by other materials, both inorganic, such as rock wool, wollastonite and fiberglass and organic materials such as aramid fibers and carbon fibers and metallic such as copper, tin, iron, aluminium and steel powders or fibers and other metals or metal alloys such as bronze and brass. The binder is in general a thermosetting polymer, for example, based on phenolic resins. Various materials are used as a filler such as barite (barium sulfate), calcium carbonate, talc, magnesium oxide, vermiculite; as abrasives, zirconium silicate, zirconium oxide, alumina, silicon carbide, mica; as friction modifiers metal sulfides such as molybdenum disulfide, iron sulfides, copper, tin, graphite and/or coke. Other classes of materials are then added in smaller percentages such as, for example, rubber in powder or granule form, “friction dust”, other organic materials.
EP0151185 teaches a material, and a process for its manufacture, constituted by an iron powder coated at least in part with tin, which is intended to produce magnetic cores for electrical apparatuses by means of sintering.
U.S. Pat. No. 8,536,244 relates to a friction material, in particular to a mixture or blend for the production of friction layers, which comprises metallic powders and/or fibers, fillers, lubricants and organic compounds, wherein are present in combination tin, tin alloys or tin compounds (typically oxides) in percentages comprised between 0.5% and 50%, and copper in percentages comprised between 0.001% and about 5%. Furthermore, the surface area of the tin particles or powders must be comprised between 1 and 5 m2/g and the tin can be as a whole or in part constituted by a surface coating applied over a rounded (spherical shaped) metallic core made of iron. In practice, this document suggests the use in friction material compositions of rounded particles having an iron core and a shell of tin or its alloys in combination with a certain content of copper, possibly being present only in traces.
However, in order to preserve the environment and to avoid possible damage to human health, increasingly more often various national and international regulations require the use of friction materials that are not only free of asbestos and heavy metals but also with a reduced or zero copper content.
The friction material according to U.S. Pat. No. 8,536,244 is furthermore very expensive to produce.
There is therefore a need in the art to provide a friction material that is free of asbestos, heavy metals (such as cadmium or antimony) and copper, having good performance, reduced production costs and that is capable of reducing the wear of the friction elements manufactured with it and especially of the elements (“friction partners”) that cooperate with friction elements, particularly when these are made of cast iron or steel.